Prep the oven and pan: Heat the oven to 375°F (190°C). Grease a 9x13-inch baking dish or similar casserole with a little butter or oil.

Cook the pasta: Bring a large pot of salted water to a boil. Cook fettuccine until just shy of al dente (about 1–2 minutes less than package directions). Reserve 1 cup of pasta water, then drain.

Season and cook the chicken: Pat chicken dry and season with salt, pepper, and Italian seasoning. Heat olive oil in a skillet over medium-high. Cook chicken 4–6 minutes per side, until golden and cooked through (165°F/74°C). Rest 5 minutes, then slice or cube.

Make the alfredo base: In the same skillet over medium heat, melt butter. Add garlic and cook 30–60 seconds until fragrant. If using flour, whisk it in and cook 30 seconds to form a light roux.

Add cream and simmer: Slowly whisk in heavy cream (and chicken broth if using). Bring to a gentle simmer. Season with salt, pepper, and a pinch of red pepper flakes if you like a little heat.

Melt in the cheese: Reduce heat to low. Stir in parmesan by handfuls until melted and smooth. Adjust thickness with splashes of reserved pasta water. You want a glossy sauce that lightly coats a spoon.

Combine: Add the drained pasta and chicken to the sauce. Toss until everything is evenly coated. If it looks too tight, add more pasta water. Taste and adjust seasoning. A small squeeze of lemon can brighten the richness.

Assemble in the dish: Transfer to the prepared baking dish. Top with a light layer of parmesan and mozzarella for a golden finish.

Bake: Bake 12–18 minutes, until the top is bubbling and lightly browned at the edges. For more color, broil 1–2 minutes, watching closely.

Rest and serve: Let sit 5–10 minutes so the sauce settles. Garnish with chopped parsley and extra black pepper. Serve warm.